ok your php info looks ok... maybe post this question in the phpnuke forum (if they have one - i hope so) coz i guess they r far more experienced with problems concerning their software... if u cannot get it working, have a look at some alternatives like postNuke, xoops, etc. have a look at hotscripts.com, php section, content managment / portal systems .... u should find a lot - sometimes rather similar to phpNuke.
However, it's hard to determine what's wrong since the error message is not really precise - so ask the people of phpNuke, i guess they know the possible reasons for "Access Denied" errors... hth